In motor vehicles whose drivetrain comprises an automated change-speed transmission and an automated starting clutch, automatic shift processes are carried out for starting and for gearshifts by means of an electronic control unit which detects various operation-relevant sensor data, such as engine speed, transmission input speed, accelerator pedal position, driving speed and shifting intention among others, and after evaluating them, initiates clutch disengagement and engagement and a change of transmission ratio by means of correspondingly controllable actuator systems.
Automated change-speed transmissions have already proved their worth as very effective geared transmission systems in motor vehicles. However, in contrast to automatic transmissions that change under load, and apart from dual-clutch systems in which gears are shifted by two clutches acting with overlap in a sequential change, they are characterized by an interruption of the traction force because the starting clutch is always disengaged during the shifting process.
Under difficult ground conditions, for example on difficult terrain or in general when the ground surface is slippery, motor vehicles designed in this way can behave problematically if an automatic gearshift is initiated suddenly while driving. For example when driving on sand or on an icy road with slipping wheels, a shift can cause the vehicle to stop or become stuck when the traction force is interrupted. If the automatic shift can be carried out at all and the clutch is then closed again, the slippage is possibly increased with the result of severe wheel-spin of the driven wheels. This can sometimes bring the movement of the vehicle to an undesired stop.
To prevent that, transmission control systems are already known with which no automatically initiated shifts are carried out, if it has been recognized that wheel-slip is taking place on at least one wheel of the vehicle. This can be done, with the help of appropriate signals, by a brake system or by the drive engine in that actuation of a vehicle stabilizing system, for example a drive slip regulation (ASR) is communicated to the transmission control unit, and/or in that the wheel speeds or wheel rotational speeds of the driven and non-driven wheels are compared with one another.
Furthermore, motor vehicles with all-wheel drive that is permanent or which can be activated are known. All-wheel drive is advantageous due to improved traction, especially on difficult terrain or on a slippery road. In all-wheel drive vehicles, however, wheel-slip recognition as the criterion for suppressing automatic shift processes can sometimes prove ineffective. It is true that basically, in the case of permanent all-wheel drive and all-wheel drive that can be activated and operated with variable drive torque, wheel rotational speed comparisons can be used to determine wheel slip. Often, however, precisely in difficult ground conditions, differential locks are activated in addition to the all-wheel drive. In vehicles with all-wheel drive combined with active differential locks on all the driven axles, as a rule all the wheels necessarily have the same speed or rotational speed and conventional wheel rotational speed comparisons can no longer be used to determine whether or not wheel slip is taking place. Despite this, there is still a need not to carry out any automatic shifts, if slip of the vehicle's wheels is occurring. Until now no control methods have been known which overcame this problem satisfactorily.
Basically, an operating element could be provided which enables the driver to switch off an automatic shift mode. However, this would presuppose that the driver anticipates the risk of wheel slip and switches off the automatic shift mode before the control system can initiate a shift operation, possibly when wheel-spin is about to begin. There is also the risk that in an occasional difficult driving situation the driver would have too much to do, or would at least be distracted, if he had to operate an additional switch.
It would also be conceivable to have a sensor system that analyzed the vehicle and its surroundings with reference to a relative position or relative movement, to determine whether, when its driven wheels are turning, the vehicle is actually moving ahead at a speed corresponding to the wheel speed. However, that would entail relatively high costs and implementation complexity.